The removal of photoresist coatings (“resist strip”) is a critical process in semiconductor manufacturing and has historically been performed in a batch type processing mode with 25 to 100 wafers being immersed in a mixture of sulfuric acid and peroxide (SPM) for up to 20 minutes. As semiconductor devices shrink in size, defectivity is a significant challenge. To address the high defectivity associated with batch processing, industry focus has switched to developing and using single wafer type processes.
For many reasons, both economic and technical, single wafer SPM processes operate at higher temperatures (170°-220° C.) than batch processes (120°-150° C.). To make single wafer SPM processing economically feasible, the resist strip time must be reduced from 10 minutes to ideally less than 2 minutes. This can be achieved with the higher process temperatures.
High dose ion implant resist strip (HDIRS) is also a driving factor for high temperature single wafer SPM processing, as the crust created when the photoresist is bombarded by high-energy ions is notoriously difficult to remove. A key advantage for single wafer processing is that higher temperatures can be utilized to strip resist coatings. Higher process temperatures have been shown to significantly improve resist strip performance for higher dosed resists (e.g. 1×1014 atoms/cm2).
There are several difficulties that must be managed when using higher temperature SPM. For example, material selection for processing chamber materials is restricted to those that would be stable in contact with 220° C. SPM. Another concern is that significant silicon nitride and silicon dioxide film loss is measured at temperatures above 170° C. Typically, the process should strip photoresist without any loss of silicon nitride (Si3N4) or silicon dioxide (SiO2).
Another complication is the high level of mist generation in the process chamber. This is a challenge to make multi-chemical processing possible. SPM processing is typically followed by a Standard Clean 1 (SC1) step to remove particles from the wafer. SC1 is a mixture of water, ammonium hydroxide and hydrogen peroxide, and any residual sulfuric acid mist in the chamber will react with ammonium hydroxide vapor emanating from the SC1 to form ammonium sulfate solid defects. Therefore, the presence of SPM mist during an SC1 process creates a defectivity challenge resulting from the two chemicals forming an undesirable precipitate that could be deposited on the wafer, e.g., H2SO4+NH4OH═NH4SO4+H2O.
Lastly, while efficient photoresist stripping rates necessitate elevated temperatures, the formation of Caro's acid is an endothermic reaction. This deprives the process of heat that is essential to high performance resist strip procedures. While it is possible to apply a heat source to the SPM as it is applied to a wafer (for example, externally applied steam), this tends to greatly increase sulfuric acid aerosols within the chamber. For the reasons explained in the paragraph above, this is highly detrimental to final component yield rates.
There is thus a need for an improved method of increasing the temperature of a process solution to improve photoresist stripping, while simultaneously reducing negative processing side effects.